Cellulosic structure and process for producing same



Patent ed Feb. 27, 1940 UNITED STATES PATENT OFFICE- CEILULO SIOSTRUCTURE AND PBbOESS FOR PRODUCING SADIE Frederick Madison Meigs,Wilmington, Del., as-

signor to E. I. du Pont de Nemours & Company, Wilmington, Del., acorporation of Dela ware No Drawing. Application May 9, 1939,

- Serial No. 272,642 a Claims. (01. sir-es) In the manufacture ofcellulosic pellicles of the I type precipitated from aqueous alkalinecellu losic solutions, such as regenerated cellulose pellicles,including sheets or films, caps, bands, continuous tubing, artificialstraw and the like, it has long been customary to incorporate into thecellulosic pellicle a softening agent in order to maintain it'in aflexible condition. Such cellulosic pellicles, when free from asoftening agent,

are known to be quite brittle, and while a softener-free pellicle mayfind certain uses, a softened and therefore flexible sheet or film ismuch more generally useful. Heretofore, as softeners for regeneratedcellulose, and similar water-sensitive cellulosic pellicles, the art hasapplied such substances as ethylene glycol, propylene glycol, diethyleneglycol, formamide, glycerol, and the like, but of these, by far the mostimportant is glycerol which has hitherto known no equal as a softeningagent of general utility for producing flexible, transparent, anddurable water-sensitive cellulosic pellicles such as those ofregenerated cellulose. Many attempts have been made to find a softenerfor water-sensitive cellulosic pellicles which will be as cheap as andwhich will have the generally useful softening characteristics ofglycerol. After continued use of previ- 'ously discovered softeningagents they were found to be deficient in some respect and thereforecould not be generally substituted for glycerol as a softening agent forregenerated cellulose and like water-sensitive cellulosic pellicles.

Glycerol is obtained commercially primarily as a by-product from thesoap industry. As a byproduct, the quantity may be limited and henceglycerol issubject to market fluctuations of supply as well as price,Glycerol has been particularly high-priced and difllcultly obtainableduring periods of war .when large quantities are used in the productionof munitions. The glycols, mentioned above as useful cellulosesofteners, are for the most part obtained by a relatively expensiveprocess of synthesis. The glycols are also relatively volatile and aretherefore unsatisfactory for use in softening some types of cellulosicpellicles. For an industry consuming large amounts of these materials,therefore, the. de-

velopment of less expensive equivalents or means I rials will mean amarked economy inproduction.

It is, therefore, an object of this invention to provide a new anduseful method forproducing softened water-sensitive cellulosic materialspossessing desirable physical properties. It is a further object of thisinvention to provide watersensitive cellulosic materials having asoftening agent associated therewith, which softening agent will notinterfere with any subsequent treatment of the cellulosic pellicles,such as the application of surface coatings, ink, coloring matter,adhesives, or the like. It is a still further object of this inventionto provide a rigid.

flexible, transparent, durable cellulosic pellicle, such as a sheet,film, ortube of regenerated cellulose in which the softening agentconsists in whole or in part of a water-soluble polyhydric ester ofcarbamic acid. Other objects of the invention will appear hereinafter.

By the term rigidity or its equivalent as used herein is meant thatproperty which is the opposite of .limpnes s; in other words, rigidityin the sense, that the pellicle can be advanced in a wrapping-machine orsimilar apparatus without curling, crumpling or otherwise fouling themachine.

The term durability is used to define resistance to shock or roughhandling. Thus, for ex-, ample, bags may be fabricated from the pellicleand filled with some standard material such as a certain weight .ofdried beans, and the package sealed, whereupon the durability may bemeasured against a known standard by dropping the package under standardconditions and noting the resistance of the wrapper to breakage.

The objectsof this invention can be attained, in general, by treating aregenerated cellulose, or a like water-sensitive cellulosic pelliclewith a softening agent comprising a water-soluble polyhydric ester ofcarbamic acid having the following structural formula:

RO-CONH2 in which R. is an alkylol radical containing from 2 to 5 carbonatoms. As specific examples of suclr compounds may be mentioned:glycerol monocarbamate, ethylene glycol monocarbamate, ,dimethyldimethylol methane monocarbamate, 2,3-butylene glycol monocarbamate,iso-butylene" glycollmonocarbamate, and trimethylene glycolmonocarbamate. The excess softening agent is then removed from thepellicle and the pellicle so treated is dried.

, In the manufacture of regenerated cellulose sheets or films, as forexample by the viscose process, the cellulosic dispersion is formed intoa sheet or film by passing the same through suitable apparatus intocoagulating and/or regenerating baths from which it is finally obtainedin the form of a continuous cellulosic sheet which is customarily led ina continuous manner through a series of purifying, bleaching, andwashing operations. Just prior to, the drying operation which is alsousually continuous, coordinating with the sheet forming and purificationoperations, the film is passed through a bath containing a softeningagent in such concentration that after the excess l quid has beenremoved from the surface of the sheet by suitable squeeze rolls and theexcess moisture removed by passage through the dryer, the final sheetcan be wound up in a continuous fashion and will contain an appropriatepredetermined amount of softening agent.

The commercially available dry regenerated cellulose film containsapproximately 4 to 8% moisture. Depending upon the particular uses forwhich the product is destined, the softening agent in the sheet or film,usually glycerol, may

.vary from about 8 to 25%, based on the combined weight of the celluloseand softening agent in the sheet.

The total amount of softener in the film is controlled primarily by thetotal amount of softening agent in the treating bath; film thickness,rate of passage through the bath, temperature of the bath, etc., mayalso contribute thereto to a certain extent. The regenerated cellulosepellicle, when reaching the treating bath, is in a highly swollen andhydrated condition and usually the cellulose of the pellicle isassociated with 300% or more of water. This highly swollen and wetpellicle, usually referred to as the gel sheet, is

impregnated with the treating bath. Because of the large amount of waterassociated with the cellulose, it is apparent that the removal of thiswater during the drying operation will concentrate the softening agentwith respect to the cellulose content of the pellicle.

Thus it is that if a regenerated cellulose pellicle containing about 15%total softener is desired as a final product, the concentration ofsoftener in the treating bath will be adjusted to approximatelyone-third that value, or about 5%. w This is true when the softeningagents are, like glycerol and the improved agents of this invention,substantially non-volatile and are not vaporized during the dryingoperation. It is obvious that if a mixed softener is used, thecomponents of which are appreciably volatile during the dryingoperation, then the softener concentration of the treatmg bath must beincreased suificiently to compgeensate for the loss thereof during thedryin s p.

It has now been discovered that esters of the type indicated above areextremely well suited as softening agents for water-sensitive cellulosicpellicles, that is, pellicles produced from aqueous alkaline cellulosicsolutions or dispersions coagulated in an acid coagulating bath,especially those of regenerated cellulose, and that they may be usedassubstitutes for, glycerol in this capacity. Furthermore, pellicles ofregenerated cellulose.

softened with thematerials of this invention possess desirableproperties which are not found in similar pellicles in which the solefiexibilizing and softening agent is glycerol.

The following examples will illustrate the practice of the invention,but it is to be understood that they are in no way limitative of theprinciples of the invention:

Example 1' v A sheet or film of gel regenerated cellulose, in which thefinal dry thickness will be about .0009", is immersed in or drawnthrough an aqueous bath containing glycerol monocarbamate, theconcentration of the softening agent being adjusted 'so that the finaldriedproduct will contain about 6.3% moisture and 16% of glycerolmonocarbamate, based on the combined weight of cellulose and softener inthe film. The sheet or film is allowed to remain in contact with thebath until thoroughly impregnated, whereupon it is removed from thebath, the excess liquid drained off or removed by suitable squeezerolls, blotters, or the like,'and the film can be dried in the usualmanner down to the final moisture content indicated above. When testedat a relative humidity of about (24C.) or even at relative humidlties aslow as 15% to 22% at ordinary temperatures, the durability of the filmis found to be at least equalto that of a similar regenerated cellulosefilm softened with about 14% glycerol;

Example II ample I, using, however, a treating bath having a compositioncapable of providing a final film containing about 6.3% moisture and 18to 19% ethylene glycol monocarbamate. In this case the final productwill be equal to or better than a similar regenerated cellulose sheet orfilm bearing 15% to 16% glycerol as softener, insofar as its physicalproperties are concerned, including transparency, durability, andflexibility under varying conditions.

Example III in Example I, exceptthat 17 to 18% dimethyl dimethylolmethane monocarbamate is substituted for the glycerol monocarbamate, Thefinal product is the equivalent of a similar film softened with 14%glycerol insofar as its physical properties are concerned, includingtransparency, flexibility and durability, and greatly superior to theyglycerinated film in its surface characteristics of slip and freedomfrom sticking.

The softening agents of this invention maybe advantageously mixed witheach other or they may be usedwith other softening agents known to theprior art. The following examples will illustrate such mixed softeningagents:

Example IV A sheet or continuous. film of gel regenerated cellulosesimilar to that of Example I is treated in a manner described in ExampleI, using, however,

a treating bath of the following approximate The ratio of glycerolmonocarbamate to glycerol in this example is. approximately 0.71:1.0.The film obtained will contain total softener in the amount 'of' about18%, based on the combined weight of the cellulose and softener, andwill exhibit excellent physical characteristics.

Example V A sheet or continuous film of gel regenerated cellulosesimilar to that of Example I is treated in the manner described inExample I, using, howi ever, a treating bath of the followingapproximate composition:

Per cent by weight Glycerol 1.0 Ethylene glycol monocarbamate 2.5

Dimethyl dlmethylol methane monocarbamate I 3.0 Water--- 93.5

Example VI A sheet or continuous film of gel regenerated cellulose suchthat the final dry thickness will be about .0009" is immersed in orpassed through a treating bath of the-following approximate composition:

Per cent by weight 'Glycerol monocarbamate 3.0 Ethylene glycol 3.1

- Water 93.9

The film is treated as described in- Example I, and yields afinalproduct containing about 6.3% moisture, 9.0% glycerol monocarbamate, and

7.5% of ethylene glycol. The ratio of glycerol monocarbamate to ethyleneglycol in the treating bath is about 1.0:1.03; the ratio in the driedfilm is about l.3:1'.0, the amount of ethylene lycol having beendecreased by loss during the drying operation. The dried film issubsequently provided with a surface coating which may be of any typedesired. Thus, for example, it may be provided with a moistureproofingcoating which may conveniently contain a cellulose derivative: apla'sticizena resin or blending agent, and a moistureproofing agent suchas a wax or wax-like material. The surface coating in this case servesthe double purpose of providing a moistureproof product and ofpreventing loss by evaporation of the relatively volatile ethylenelycol. Even when tested under such drastic conditions as described inExample I, the product is transparent, flexible, and more durable than asimilar film softened with 15% glycerol, especially with respect tosurface characteristics of slipperiness and sticking resistance.

tubes, such as may be obtained by the coagulation and/or regenerationfrom an'aqueous cellulosic dispersion in accordance with the procedurecustomary to the art. Thus, the invention comprehends the use ofregenerated cellulose pellicles such as may be obtained from the viscose5 or cuprammonium processes, as well as pellicles composed of glycolcellulose, cellulose glycollic acid, lowly esterified or lowlyetherified cellulose derivatives where there is only one ether or estergroup associated with several cellulose units, and other cellulosicstructures of similar character.

' Similarly, various types of paper, especially of the glassine type,may be used as base materials. As a matter of convenience, the inventioni has been described in terms of its application to the softening ofregenerated cellulose pellicles such as those sheets or films ofregenerated cellulose which are suitable foruseas wrapping tissue, it.being understood that thescope of the invention includes any of theother bases set forth above and that the invention is equally applicablethereto.

All of the examples given have been set forth in terms of a sheet orfilm of .gel regenerated 9| cellulose (gel regenerated cellulose beingdefined as a water-swollen regenerated cellulose which has never beendried-hence that product which is obtained from the wet end of thecasting machine). Obviously, this is the more practical wayof'practicing the invention, since the soften.

ing agent is customarily incorporated into the sheet or film while thelatter is in the gel state andduring the normal course of manufacture.Obviously, if one desires to impregnate an already dried regeneratedcellulose film with a softener of the character described, it is pas-' Ysible to rewet the dried film so as .to render it highlyswollen,.whereupon it may be treated with treating baths similar tothose described, but of composition suitable for obtaining the finalproduct desired. In the same way, any of the sheets set forth asequivalent to regenerated cellulose sheets can be substituted in thespecific examples.

The invention has been described in part in terms of the use of glycerolin combination with the softeners of the present invention. It is to beunderstood that other similar known cellulose softeners may besubstituted for the glycerol and among those may be mentioned ethyleneglycol, diethylene glycol, triethylene glycol, formamide, carbamide andother substances known to the art as softeners for water-sensitivecellulose structures.

It is also to be understood that various mixtures of the newly describedsoftening agents may be employed with single softeners of the prior artor with mixtures. of the latter, depending upon the type of final filmdesired and the purposes for which the film is to be used. I

If a colored cellulosic pellicle is desired, it may be obtained in anyof the ways commonly known in the art, including the passage of thesheet or film through' a bath containing a suitable 'dyestuif; Ifdesired, the dyestuif may be added,

to the bath'used for introducing the softening agent. In the same way,after the film has been treated with the softening agent, it may be sub-.lected toany of the customary after-treatments such as sizing orcoating, or the like, which may be customarily given to cellulosicpellicles oi the typedescribed, in just the same manner that 8- glycerolsoftened regenerated cellulose film, for example, may be treated.

The instant invention offers numerous advantages over the prior artmeans of softening cellulosic pellicles of the type described. They canbe obtained synthetically at a very low cost in comparison to the costof known softeners and.

m which R is an alkylol radical containing from 2 to 5 carbon atoms.

2. A flexible, durable,v regenerated cellulose structure, said structurecontaining, as a softener therefor, a water-soluble polyhydric ester of.

carbamic acidhaving the structure E-O-CO-NHz' in which R is an alkylolradical containing from 2 to 5 carbon atoms,

3. A flexible, durable, water-sensitive cellulosic structure, saidstructure containing, as a softener therefor, glycerol monocarbamate.

4. A flexible, durable, water-sensitive cellulosic structure, saidstructure containing, as a softener therefor, ethylene glycolmonocarbamate.

5. A flexible, durable, water-sensitive cellulosic structure, saidstructure-containing, as a softener therefor, dimethyl dimethylolmethane monocarbamate.

FREDERICK MADISON MIEIGS.

